Cavity optomagnonics with magnetic textures: coupling a magnetic vortex to light

Optomagnonic systems, where light couples coherently to collective excitations in magnetically ordered solids, are currently of high interest due to their potential for quantum information processing platforms at the nanoscale. Efforts so far, both at the experimental and theoretical level, have focused on systems with a homogeneous magnetic background. A unique feature in optomagnonics is however the possibility of coupling light to spin excitations on top of magnetic textures. We propose a cavity-optomagnonic system with a non homogeneous magnetic ground state, namely a vortex in a magnetic microdisk. In particular we study the coupling between optical whispering gallery modes to magnon modes localized at the vortex. We show that the optomagnonic coupling has a rich spatial structure and that it can be tuned by an externally applied magnetic field. Our results predict cooperativities at maximum photon density of the order of $\mathcal{C}\approx10^{-2}$ by proper engineering of these structures.Comment: 16 pages, 11 figures, published versio

Method-based caching in multi-tiered server applications

Abstract In recent years, application server technology has become very popular for building complex but mission-critical systems such as Web-based E-Commerce applications. However, the resulting solutions tend to suffer from serious performance and scalability bottlenecks, because of their distributed nature and their various software layers. This paper deals with the problem by presenting an approach about transparently caching results of a service interface\u27s read-only methods on the client side. Cache consistency is provided by a descriptive cache invalidation model which may be specified by an application programmer. As the cache layer is transparent to the server as well as to the client code, it can be integrated with relatively low effort even in systems that have already been implemented. Experimental results show that the approach is very effective in improving a server\u27s response times and its transactional throughput. Roughly speaking, the overhead for cache maintenance is small when compared to the cost for method invocations on the server side. The cache\u27s performance improvements are dominated by the fraction of read method invocations and the cache hit rate. Our experiments are based on a realistic E-commerce Web site scenario and site user behaviour is emulated in an authentic way. By inserting our cache, the maximum user request throughput of the web application could be more than doubled while its response time (such as perceived by a web client) was kept at a very low level. Moreover, the cache can be smoothly integrated with traditional caching strategies acting on other system tiers (e.g. caching of dynamic Web pages on a Web server). The presented approach as well as the related implementation are not restricted to application server scenarios but may be applied to any kind of interface-based software layers

Generation and subwavelength focusing of longitudinal magnetic fields in a metallized fiber tip

We demonstrate experimentally and numerically that in fiber tips as they are used in NSOMs azimuthally polarized electrical fields (|E$_{\text{azi}}$|$^2$/|E$_{\text{tot}}$|$^2$ $\approx$ 55% $\pm$ 5% for 1.4\mu m tip aperture diameter and \lambda$_0$ = 1550nm), respectively subwavelength confined (FWHM $\approx$ 450nm $\approx$ \lambda$_0$/3.5) magnetic fields, are generated for a certain tip aperture diameter (d = 1.4\mu m). We attribute the generation of this field distribution in metal-coated fiber tips to symmetry breaking in the bend and subsequent plasmonic mode filtering in the truncated conical taper.Comment: 11 pages, 6 figure

Optomechanische Kristalle für Netzwerke in dünnen Siliziumschichten - Verstimmbarkeit, Einfluss von Fehlordnung und 2D Strukturentwürfe für Tieftemperaturexperimente

In this thesis approaches for the design of optomechanical crystal cavities are presented that tackle the challenge to construct a tunable mechanical resonator in the GHz regime and an optomechanical crystal device that is suitable for quantum optomechanics at dilution refrigerator temperatures. The first part demonstrates the design and steps towards the experimental realization of an optomechanical crystal cavity with a dynamically tunable GHz-frequency mechanical mode. This includes a concept to tune mechanical defect modes with applied capacitive forces and an implementation technique to maximize exertable forces. To generate the acoustic cavity, a dimerization of an optomechanical unit cell is used to fold acoustic X-point modes back to the Γ-point of the band structure, which lifts the symmetry restrictions for substantial optomechanical couplings. The developed design shows how optical and acoustic elements can be split to make both elements separately optimizable, while still forming a single optomechanical crystal defect. Structural stress releasing elements for a thin film silicon platform are analyzed that allow to implement both large scale capacitors and delicate optomechanical crystal cavities within the same device. The following part presents the development of a novel two dimensional optomechanical crystal cavity that is suited for quantum optomechanical experiments at milli-Kelvin temperatures. The two dimensional design increases the thermal connectivity to the environment compared to conventional optomechanical nanobeam devices. It still exhibits optomechanical coupling strengths that reach similarly excellent values as in a nanobeam cavity by the use of breathing type acoustic modes at a line defect. The fabrication complexity of the structure is reduced by the a priori use of rounded elements. The final design is fabricated and experimentally demonstrated. Its integration into a dilution refrigerator setup is discussed. Finally, it is studied how different fabrication imperfections affect the optical quality factors of the previously introduced cavities. The reduction of parasitic losses is of enormous interest for the development of devices that can perform coherent quantum operations. An example for such a loss channel is the photon leakage from an optomechanical cavity by scattering from perturbations of the optomechanical crystal material. For a targeted reduction of these mechanisms, it is key to quantify the impact of different imperfections. Both systematic and random perturbations are analyzed and techniques to avoid them or reduce their influence are discussed. A comparison to other photonic crystal cavity devices is made and a heuristic indicator for the robustness of a general defect cavity geometry is introduced.In dieser Arbeit werden zwei neue Varianten von Defekt-Kavitäten in optomechanischen Kristallen vorgestellt. Die Neuheit der ersten präsentierten Struktur ist die Verstimmbarkeit der akustischen Resonanzfrequenz im GHz Bereich. Die zweite Struktur ist für quantenoptome chanische Experimente unter Umgebungstemperaturen im Millikelvinbereich geeignet. Im ersten Teil wird das Design und Schritte hin zu einer experimentellen Realisierung einer optomechanischen Kristall Kavität mit dynamisch verstimmbarer mechanischer Mode im GHz Bereich vorgestellt. Dabei wird auf das der Verstimmbarkeit zugrunde liegende Konzept der Kraftausübung durch integrierte kapazitive Elemente eingegangen und darauf wie diese erzeugte Kraft maximiert werden kann. Zur Erzeugung der akustischen Kavität wird eine X-Punkt-Mode genutzt, die mittels Dimerisation zurück auf den Γ-Punkt der Bandstruktur gefaltet wird. Dies geschieht um die symmetriebedingten Einschränkungen der optomechanischen Kopplung solcher Moden zu umgehen. Im vorgestellten Design wird zudem eine Methode angewandt, die die geometrischen Elemente zur Erzeugung der optischen und akustischen Defektkavität räumlich trennt und so separat optimierbar macht, obwohl diese weiterhin eine gemeinsame optomechanische Kavität bilden. Um sowohl die empfindlichen Element der Defekt-Kavität, als auch die vergleichsweise großen kapazitiven Elemente, in einer einzigen Struktur unterbringen zu können, werden verschiedene strukturelle Geometrien analysiert,die die, dem Material inhärente, Spannung des Siliziumsfilms kompensieren können. Im folgende Teil wird eine neuartige 2D optomechanische Kristall Kavität präsentiert, die für quantenoptomechanische Experimente in einem Mischungskryostaten entwickelt wurde. Der Vorteil des zweidimensionalen optomechanischen Kristalls gegenüber konventionellen Strukturen besteht in der besseren thermischen Konnektivität zur Umgebung. Die optomechanische Kopplung erreicht dabei vergleichbar hohe Werte zu den etablierten eindimensionalen Kavitäten, indem für den mechanischen Resonator eine atmende akustische Mode eines Zeilendefekts im Kristall verwendet wurde. Die Komplexität der Herstellung im Vergleich zu anderen 2D Designs konnte durch die Verwendung abgerundeter Formen reduziert werden. Die Struktur wurde hergestellt und eine erste experimentelle Charakterisierung vorgenommen. Die folgenden Schritte zur Integration in einen Mischungskryostaten werden vorgestellt. Abschließend folgt eine Analyse des Effekts verschiedener herstellungsbedingter Unregelmäßigkeiten und Fehler auf den Q-Faktor der optischen Mode der bereits vorgestellten 2D optomechanischen Kavität. Die Reduzierung von parasitären Verlusten ist für die Entwicklung von Strukturen, die zur Durchführung kohärenter Quantenoperationen verwendet werden sollen, von großer Bedeutung. Ein Beispiel für diese ist der Schwund von Photonen ausder Defekt-Kavität durch die Streuung an Störungen der optomechanischen Struktur. Um solche Mechanismen gezielt reduzieren zu können ist die genaue Quantifizierung ihrer Auswirkungen unerlässlich. Es werden sowohl systematische als auch chaotische Störungen des Herstellungsprozesses betrachtet und Vermeidungsstrategien beziehungsweise Techniken zur Reduktion des störenden Einflusses diskutiert. Ein Vergleich zu anderen Defekt-Kavitäten in photonischen Kristallen wird angestellt und ein heuristischer Indikator für die generelle Anfälligkeit eine Defekt-Kavität gegen Störungen eingeführt

Position-squared coupling in a tunable photonic crystal optomechanical cavity

We present the design, fabrication, and characterization of a planar silicon photonic crystal cavity in which large position-squared optomechanical coupling is realized. The device consists of a double-slotted photonic crystal structure in which motion of a central beam mode couples to two high-Q optical modes localized around each slot. Electrostatic tuning of the structure is used to controllably hybridize the optical modes into supermodes which couple in a quadratic fashion to the motion of the beam. From independent measurements of the anti-crossing of the optical modes and of the optical spring effect, the position-squared vacuum coupling rate is measured to be as large as 245 Hz to the fundamental in-plane mechanical resonance of the structure at 8.7MHz, which in displacement units corresponds to a coupling coefficient of 1 THz/nm$^2$. This level of position-squared coupling is approximately five orders of magnitude larger than in conventional Fabry-Perot cavity systems.Comment: 11 pages, 6 figure

Design of tunable GHz-frequency optomechanical crystal resonators

We present a silicon optomechanical nanobeam design with a dynamically tunable acoustic mode at 10.2 GHz. The resonance frequency can be shifted by 90 kHz/V^2 with an on-chip capacitor that was optimized to exert forces up to 1 µN at 10 V operation voltage. Optical resonance frequencies around 190 THz with Q-factors up to 2.2 × 10^6 place the structure in the well-resolved sideband regime with vacuum optomechanical coupling rates up to g_0/2π = 353 kHz. Tuning can be used, for instance, to overcome variation in the device-to-device acoustic resonance frequency due to fabrication errors, paving the way for optomechanical circuits consisting of arrays of optomechanical cavities

Direct laser-written optomechanical membranes in fiber Fabry-Perot cavities

Integrated micro and nanophotonic optomechanical experiments enable the manipulation of mechanical resonators on the single phonon level. Interfacing these structures requires elaborate techniques limited in tunability, flexibility, and scaling towards multi-mode systems. Here, we demonstrate a cavity optomechanical experiment using 3D-laser-written polymer membranes inside fiber Fabry-Perot cavities. Vacuum coupling strengths of ~ 30 kHz to the fundamental megahertz mechanical mode are reached. We observe optomechanical spring tuning of the mechanical resonator by tens of kHz exceeding its linewidth at cryogenic temperatures. The extreme flexibility of the laser writing process allows for a direct integration of the membrane into the microscopic cavity. The direct fiber coupling, its scaling capabilities to coupled resonator systems, and the potential implementation of dissipation dilution structures and integration of electrodes make it a promising platform for fiber-tip integrated accelerometers, optomechanically tunable multi-mode mechanical systems, or directly fiber-coupled systems for microwave to optics conversion.Comment: 10 pages, 5 figure